The communication speeds of wireless LAN systems standardized as the IEEE802.11 (to be referred to as 802.11 hereinafter) standards have increased with the advent of 802.11b, 802.11g, and the like. Presently, 802.11n for 100 Mbps or more is being standardized.
802.11n has proposed a frame aggregation method as a technique that improves the throughput. Frame aggregation is a technique that reduces the overhead of a header or the like by transmitting a plurality of data by binding (multiplexing) them into one header.
This frame aggregation method allows 802.11n to transmit data by using a communication frame having a data length larger than those of systems such as 802.11a, 802.11b, and 802.11g.
Various techniques have been proposed in communication systems using the frame aggregation method.
In one technique, a plurality of MAC frames to be aggregated into a PHY frame are selectively aggregated in accordance with the attributes of the MAC frames (Japanese Patent Laid-Open No. 2006-246539).
In another technique, the data transmission period is divided into an active period and inactive period, and the sizes of packets to be aggregated in the active period and inactive period are made different from each other (Japanese Patent Laid-Open No. 2005-176367).
In still another technique, the size of packets to be aggregated is changed based on the fluctuations of a propagation path to be used in data transmission (Japanese Patent Laid-Open No. 2006-173867).
Also, 802.11n is scheduled to support the three types of PHY frame formats, that is, the Legacy mode, Mixed mode, and Greenfield mode.
The Legacy mode is a frame having the same structure as that of 802.11a and 802.11g. Even an apparatus (to be referred to as a Legacy apparatus hereinafter) that supports 802.11a and 802.11g but cannot support 802.11n can interpret this frame format including data.
The Mixed mode is a frame in which the preamble of 802.11n follows a preamble which apparatuses of 802.11a and 802.11g can interpret. This frame has compatibility with 802.11a and 802.11g.
The Greenfield mode is a frame which only an apparatus (to be referred to as an 11n apparatus hereinafter) corresponding to 802.11n can interpret.
Thus, 802.11n is expected to achieve backward compatibility with 802.11a and 802.11g.
802.11n is also scheduled to include the techniques of QoS (Quality of Service) introduced by 802.11e.
As described above, 802.11n can improve the throughput by using the frame aggregation method. On the other hand, the frame aggregation method may have influence on data transmission of peripheral apparatuses. That is, as the number of frames to be aggregated increases, the communication frame length increases, and this prolongs the time during which an 11n apparatus occupies the band. This probably decreases the throughput because the time during which peripheral apparatuses can transmit data shortens.
Furthermore, 802.11n changes the length of a preamble portion in accordance with the type of communication frame, so the band occupation time changes in accordance with the type of communication frame to be used. Accordingly, when a communication frame having a large preamble length is used, the band occupation time is prolonged. This presumably decreases the throughput of data transmission by peripheral apparatuses as in the case of frame aggregation.
This problem can be a serious problem when performing QoS control (priority control). For example, if an 11n apparatus starts forwarding data by using frame aggregation while a Legacy apparatus is performing video data transmission (having a high priority) by QoS control of 802.11e, the communication quality of the Legacy apparatus probably deteriorates.